Olefin based polymers are widely used in various applications because they are chemically inert, have a low density, and are cost effective. Applications include, for example, adhesives, films, fibers, molded parts, and combinations thereof. While these polymers are solid at room temperature, they are often produced and processed as melts. The last step in many manufacturing processes for such materials is converting the polymer melt into easily handled granules. Granules—pellets being one type—are advantageous as they can be easily packaged, transported, weighed, batched, and reprocessed.
Granulation, or pelletization, may be achieved in a variety of ways. For low viscosity materials (i.e., those polymers having a viscosity less than 100 cP at 190° C. and determined according to ASTM D-3236), the polymer melt is applied onto a cooling surface, the melt is cooled into a solid, and the solid is recovered as flakes, pastilles, briquettes, granules, or other suitable forms. Often, however, the granulation step is skipped altogether for low viscosity melts, and the melts are packaged in transportable melt tanks. For higher viscosity polymers, granulation generally involves forcing the polymer melt through a die and cooling and cutting the resulting strands into pellets. When these higher viscosity polymers have suitably high molecular weights, it is possible to make clean cuts resulting in discrete, commercially acceptable pellets at a wide range of processing temperatures.
For certain lower viscosity polymers, however, acceptable pellets are only formed at relatively low temperatures. Previously, these low temperatures were achieved by processing the polymers in surface-cooled extruders, which are able to cool the polymer melt through the metal housing of an extruder using a circulated refrigerant. For large volume industrial processes, however, this method is not feasible because the amount of heat that needs to be removed from the polymer to obtain acceptable pellets is directly proportional to the amount of polymer, while the amount of heat that can actually be removed is proportional to the surface area of the extruder. Because the former depends upon volume (and the cube of linear dimensions) while the latter depends upon area (and the square of linear dimensions), scaling up for larger processes requires either very long periods of cooling time or multiple smaller extruders in series to handle the larger volume of polymer material.
It would be desirable, then, to develop a more efficient method for cooling low viscosity polymers so that the polymers can be pelletized. The present invention addresses these issues by providing a method for evaporative cooling of low viscosity, semi-crystalline polymers by removing solvent in a vacuum assisted devolatilization device for a time sufficient to allow substantially complete crystallization of the polymers.